Stabilized solid garnet electrolyte and methods thereof

ABSTRACT

An air stable solid garnet composition, comprising: 
     a bulk composition and a surface protonated composition on at least a portion of the bulk composition as defined herein, and the protonated surface composition is present on at least a portion of the exterior surface of the bulk composition at a thickness of from 0.1 to 10,000 nm. Also disclosed is a composite electrolyte structure, and methods of making and using the composition and the composite electrolyte structure.

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 62/084,907 filed on Nov. 26, 2014the content of which is relied upon and incorporated herein by referencein its entirety.

The entire disclosure of any publication or patent document mentionedherein is incorporated by reference.

BACKGROUND

The disclosure relates to a solid garnet electrolyte article and tomethods of making and using the solid garnet electrolyte article in anenergy storage device.

SUMMARY

In embodiments, the disclosure provides a solid garnet electrolytearticle and to methods of making and using the solid garnet electrolytearticle in an energy storage device.

BRIEF DESCRIPTION OF DRAWINGS

In embodiments of the disclosure:

FIG. 1 is a schematic showing applicable acid treatment and regenerationsteps.

FIG. 2 is a schematic in cross section showing a test cell assembly(200) incorporating a lithium metal anode (150) in contact with a solidgarnet electrolyte (110) of the disclosure.

FIG. 3 compares an operating cell voltage for acid treated garnetpellets (diamonds; 310) and untreated garnet pellets (squares, 300)(control) as a function of current density.

FIG. 4 shows an exemplary operating cell voltage over time for an acidtreated garnet pellet at high current density of 1.0 mA/cm².

FIG. 5 shows a TGA trace comparing an acid untreated garnet control(500) sample and an acid treated garnet (510) sample after ambientmoisture exposure.

DETAILED DESCRIPTION

Various embodiments of the disclosure will be described in detail withreference to drawings, if any. Reference to various embodiments does notlimit the scope of the invention, which is limited only by the scope ofthe claims attached hereto. Additionally, any examples set forth in thisspecification are not limiting and merely set forth some of the manypossible embodiments of the claimed invention.

Definitions

“Air,” “ambient air,” “ambient atmosphere,” “atmospheric air”, or liketerms refers to chemical constituents occurring in earth's atmosphere,for example, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93%argon, 0.039% carbon dioxide, and small amounts of other gases. Humidair also includes water vapor.

“Compositionally stable,” or like terms refers to the disclosed solidgarnet composition, which composition once formed does not significantlychange its composition, for example, when exposed to ambient atmospherefor several hrs or more.

“Air stable,” or like terms also refers to the disclosed solid garnetcomposition, which composition once formed does not significantly changeits composition, for example, when exposed to air or ambient atmospherefor several hrs or more.

“Insensitive to carbon dioxide,” or like terms or phrases also refers tothe disclosed solid garnet composition, which composition once formeddoes not significantly change its composition, for example, when exposedto air or ambient atmosphere containing carbon dioxide for several hrsor more.

“Include,” “includes,” or like terms means encompassing but not limitedto, that is, inclusive and not exclusive.

“About” modifying, for example, the quantity of an ingredient in acomposition, concentrations, volumes, process temperature, process time,yields, flow rates, pressures, viscosities, and like values, and rangesthereof, or a dimension of a component, and like values, and rangesthereof, employed in describing the embodiments of the disclosure,refers to variation in the numerical quantity that can occur, forexample: through typical measuring and handling procedures used forpreparing materials, compositions, composites, concentrates, componentparts, articles of manufacture, or use formulations; through inadvertenterror in these procedures; through differences in the manufacture,source, or purity of starting materials or ingredients used to carry outthe methods; and like considerations. The term “about” also encompassesamounts that differ due to aging of a composition or formulation with aparticular initial concentration or mixture, and amounts that differ dueto mixing or processing a composition or formulation with a particularinitial concentration or mixture.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

The indefinite article “a” or “an” and its corresponding definitearticle “the” as used herein means at least one, or one or more, unlessspecified otherwise.

Abbreviations, which are well known to one of ordinary skill in the art,may be used (e.g., “h” or “hrs” for hour or hours, “g” or “gm” forgram(s), “mL” for milliliters, and “rt” for room temperature, “nm” fornanometers, and like abbreviations).

Specific and preferred values disclosed for components, ingredients,additives, dimensions, conditions, times, and like aspects, and rangesthereof, are for illustration only; they do not exclude other definedvalues or other values within defined ranges. The composition andmethods of the disclosure can include any value or any combination ofthe values, specific values, more specific values, and preferred valuesdescribed herein, including explicit or implicit intermediate values andranges.

As ceramic lithium electrolytes have shown practical conductivities inexcess of 10⁻⁴ S/cm are now well known (see Knauth, Solid State Ionics(2009) 180, 911-916), there is an interest in other ceramic Li-ionelectrolytes that can enable high energy cell structures. The newdesigns rely on the ceramic electrolyte to separate incompatible anodeand cathode chemistries through hermetic isolation. For example, a newapproach to lithium-sulfur cell chemistry combines an aqueous sulfurcathode with a lithium metal anode (see Visco, et al., U.S. Pat. No.8,828,574).

To enable a Li-metal anode, the ceramic electrolyte must be stableagainst lithium metal contact. Presently, garnet phase electrolytes suchas Li₇La₃Zr₂O₁₂ (LLZO) have demonstrated both high lithium-ionconductivity (greater than 10⁻⁴ S/cm) and stability against Li-metalcontact (Weppner, et. al., U.S. Pat. No. 8,658,317). Unfortunately,garnet materials are susceptible to forming an insulating film oflithium carbonate on exposure to ambient conditions (see Cheng, et. al.,Phys. Chem. Chem. Phys., (2014) 16, 18294-18300). This insulating filmcan dominate the impedance of a cell if it is not removed. Cheng, et.al., suggest polishing under inert atmosphere as a solution to removalof the insulating Li₂CO₃ film. However, after polishing, the garnetmaterial must be protected from further ambient exposure, or the lithiumcarbonate insulating film will form again. An ability to render thegarnet electrolyte structures insensitive to ambient carbon dioxide topermit ambient processing is desirable.

It is known that garnet materials can reversibly undergo H⁺/Li⁺ exchangein aqueous solutions (see Ma, C., et al., (2014), Excellent Stability ofa Lithium-Ion-Conducting Solid Electrolyte upon Reversible Li⁺/H⁺Exchange in Aqueous Solutions. Angew. Chem. DOI:10.1002/ange.201408124). When an as-synthesized garnet material isimmersed in neutral water, the pH of the aqueous phase shifts towards abasic pH due to formation of LiOH, e.g., a Li₇La₃Zr₂O₁₂ (LLZO) garnetaccording to the equation:

Li₇La₃Zr₂O₁₂ +xH₂O→Li_((7-x))H_(x)La₃Zr₂O₁₂ +xLiOH

This same reaction occurs at the surface of a garnet material handled inambient air. It is known that LiOH getters or absorbs atmospheric CO₂ toform Li₂CO₃ (see Jaunsen, J. R. (1989) “The Behavior and Capabilities ofLithium Hydroxide Carbon Dioxide Scrubbers in a Deep Sea Environment,”US Naval Academy Technical Report, USNA-TSPR-157.).

2LiOH—H₂O+CO₂→Li₂CO₃+3H₂O

or

2LiOH+CO₂→Li₂CO₃+H₂O

The end product of the reaction sequence is an insulating surface filmof lithium carbonate (Li₂CO₃). A similar reaction sequence was alsoobserved for a LiLaTiO₃ electrolyte (see Boulant, et. al., Dalton Trans.(2010) 39 3968-3975).

Since the H⁺/Li⁺ exchange is reversible, it is possible to treat thegarnet material in acid to neutralize the surface of the garnet materialand decompose the formed Li₂CO₃. For example, Li₂CO₃ can be decomposedwith HCl:

Li₂CO₃+2HCl→2LiCl+H₂O+CO₂

In embodiments, the disclosed method can effectively clean the garnetsurface of, for example, LiOH, Li₂CO₃, and like species. The disclosedmethod can also effectively regenerate a lithium ion active garnetsurface for use, for example, in an energy storage article.

In embodiments, the disclosure provides a solid garnet electrolytearticle and to methods of making and using the solid garnet electrolytearticle in an energy storage device.

In embodiments, the disclosure provides an air stabilized solid garnetcomposition and a solid electrolyte article thereof, and the disclosureprovides methods of making and using the composition and the stabilizedsolid garnet electrolyte article.

In embodiments, the disclosure provides a method for passivating a solidlithium garnet electrolyte against surface fouling by ambientatmosphere.

In embodiments, the disclosure provides a method for treating a solidlithium garnet electrolyte, which treatment method permits ambienthandling or processing of the garnet electrolyte.

In embodiments, the disclosure provides a method for regenerating,either in situ or ex situ, a passivated solid garnet electrolyte toproduce an active lithium ion surface specie suitable for use in anenergy storage device. The disclosure provides an example of in situregeneration.

In embodiments, the disclosure provides a solution treatment method of asolid garnet electrolyte that cleans Li₂CO₃ from the surface, which canlead to improved cell performance.

In embodiments, the disclosure provides a passivation method where agarnet membrane having lithium containing surface is converted to agarnet membrane product having a lithium-deficient surface, whichproduct is stable to, for example, atmospheric moisture, oxygen, andcarbon dioxide. In embodiments, the passivation method comprises an ionexchange of lithium ions of the garnet membrane with protons (i.e.,Li^(⇄)/H⁺ exchange). The resulting super-protonated membrane can behandled under ambient conditions without concern for carbonate filmformation. The super-protonated garnet can then be electrochemicallyconverted back to a lithium-rich garnet with a lithium-containingelectrode.

The present disclosure is advantaged in several aspects, including forexample: The disclosed treatment method eliminates a resistive carbonatesurface layer on the garnet substrate, which eliminated resistivecarbonate surface layer, and improves the performance of a cellincluding a garnet electrolyte.

The method of making, involving contacting the garnet electrolyte with aprotic acid, can be applied to a garnet electrolyte in numerous forms,which is impractical for alternative surface treatment methods such assurface polishing. Example garnet electrolyte forms that can be treatedaccording to the disclosed method include: a porous garnet surface(which can be useful for improving electrode contact), a thin garnetcoating (e.g., less than 20 microns thick), a thin garnet membrane(e.g., less than 200 microns thick such as 150 microns, 100 microns, 50microns, including intermediate values and ranges), and like forms.

The method of making involving acid treatment of, for example, thelithium garnet electrolyte can be accomplished with a brief exposure tothe acid, for example, from 1 to 2 hrs at ambient temperature.

Since the lithium ion species in the garnet electrolyte surface arechemically neutralized by the disclosed method, the formation of lithiumcarbonate is at least temporarily mitigated and ambient air handling ofthe garnet membrane is now possible, and simplifies cell constructionand lowers fabrication costs.

In embodiments, the present disclosure provides a solid garnetcomposition, comprising:

-   a bulk composition comprising a lithium garnet; and-   a surface composition comprising a surface protonated garnet on at    least a portion of the exterior surface of the lithium garnet,    wherein the surface protonated portion of the garnet composition is    compositionally stable in air and insensitive to carbon dioxide.

In embodiments, the disclosure provides a solid garnet composition,comprising:

-   a bulk composition consisting of or consisting essentially of a    lithium garnet; and-   a surface composition consisting of or consisting essentially of a    surface protonated garnet on at least a portion of the exterior    surface of the lithium garnet, wherein the surface protonated solid    garnet composition is compositionally stable in air and insensitive    to carbon dioxide.

In embodiments, the bulk lithium garnet can be, for example, of theformula Li₇La₃Zr₂O₁₂ (LLZO) and the surface protonated garnet can be,for example, of the formula Li_((7-x))H_(x)La₃Zr₂O₁₂, where x is from0.1 to 6.75.

In embodiments, the bulk lithium garnet can be, for example, of theformula Li_(6.75)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂ and the surfaceprotonated garnet can be, for example, of the formulaLi_((6.75-x))H_(x)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂, where x is from0.1 to 6.75.

In embodiments, the surface protonated garnet can be, for example,formed on opposite sides or faces of the acid treated solid garnetcomposition such as the large faces or small faces the treated membrane.The non-acid treated surfaces can be, for example, protected with a masklayer. In embodiments, the surface protonated garnet can be, forexample, formed on the entire outer surface of the bulk lithium garnet.

In embodiments, the disclosed solid garnet composition can be, forexample, a doped LLZO, for example, of the general formulaLi_(7-z)La_(3-y)Ga_(y)Zr_(2-z)Nb_(z)O₁₂ or of the more specific formulaLi_(6.75)La_(2.9)Ga_(0.1)Zr_(1.75)Nb_(0.25)O₁₂, having a surfacecomposition, for example, of the formula ofLi_((6.75-x))H_(x)La_(2.9)Ga_(0.1)Zr_(1.75)Nb_(0.25)O₁₂, or a compoundof the formula, Li_((7-z-x))H_(x)La_(3-y)Ga_(y)Zr_(2-z)Nb_(z)O₁₂, or acombination thereof, where x is from 0.1 to 7, y is from 0.1 to 0.3, andz is from 0.1 to 0.3.

Garnet pellet is one example of a suitable garnet. The disclosedcompositions, and methods of making and use, are suitable for all garnetmembranes irrespective of the method of making the bulk garnet so longas the garnet is exposed to ambient atmospheric conditions, including,for example, water, carbon dioxide, or like entities, since the garnet,unless processed in accordance with the present disclosure will form anon-protonic passive layer.

In embodiments, the protonated garnet on at least a portion of theexterior surface of the lithium garnet includes, for example, the entireexterior surface of the lithium garnet.

In embodiments, the protonated garnet on at least a portion of theexterior surface of the lithium garnet includes, for example, a layerhaving a thickness of from 0.1 to 100 nm.

In embodiments, the surface composition has a lithium concentration offrom 0.1 to 46 mol %.

In embodiments, the surface composition can be, for example,substantially free of an electrically insulating Li surface species, forexample, from 0 to less than from 10 mol %.

In embodiments, the surface composition comprises a surface free of atleast one of LiOH, Li₂CO₃, or a combination thereof.

In embodiments, the disclosure provides a composite electrolytestructure, comprising:

-   a protected anode structure comprising:    -   a first layer comprising a solid ceramic garnet electrolyte        composition comprising a solid garnet composition, comprising:-   a bulk composition comprising a lithium garnet; and-   a surface composition comprising a protonated garnet on at least a    portion of the exterior surface of the lithium garnet, wherein the    solid garnet composition is compositionally stable in air and    insensitive to carbon dioxide;    -   a second layer comprising a source of lithium metal, wherein the        first layer and the second layer are in physical contact, and        contained within the protected anode structure; and-   a cathode.

In embodiments, the composite electrolyte structure can furthercomprise, for example, an aqueous catholyte situated between and incontact with the protected anode structure and the cathode.

In embodiments, the cathode can be, for example, a lithium cobalt oxidecompound of the formula LiCoO₂.

In embodiments, the disclosure provides a method of making theaforementioned solid garnet solid, comprising:

-   contacting an air sensitive lithium containing solid garnet    electrolyte with a protic acid to form a solid garnet electrolyte    having a protonated surface.

In embodiments, the protonated surface solid garnet composition iscompositionally stable in air and insensitive to carbon dioxide.

In embodiments, the protic acid can selected be, for example, from atleast one of: a mineral acid, for example, HCl, H₃PO₄, HNO₃, H₂SO₄, andlike acids, or mixtures thereof, an organic acid, for example, aceticacid, or like organic acids, or mixtures thereof, or a combination ofmineral and organic acids.

In embodiments, the method of making can further comprise, for example,contacting the air stable and carbon dioxide insensitive solid garnetelectrolyte having the protonated surface with a source of lithium ionsto regenerate the air sensitive lithium containing solid garnetelectrolyte.

In embodiments, the contacting with a source of lithium ions can beaccomplished, for example, in a sealed cell structure, for example,treating the garnet electrolyte with a lithium ion source where thegarnet electrolyte is in a sealed cell structure and not on a baremembrane.

In embodiments, the source of lithium ions can be, for example, lithiummetal.

In embodiments, the disclosure provides a passivation method comprising:

-   ion exchanging the lithium ions on the surface of an air sensitive    lithium containing solid garnet membrane with protons to form an air    stable and a carbon dioxide insensitive solid garnet electrolyte    having a lithium-depleted protonated surface.

Referring to the Figures, FIG. 1 is a schematic (100) showing aspects ofthe disclosed method. In embodiments, when a membrane (110), forexample, an LLZO, is exposed to ambient atmosphere or air (115)containing, for example, carbon dioxide and water, a surface layercontaining LiOH and Li₂CO₃ (120) and minor amounts of HLZO (130) can beformed on the exposed surface, that is, products of exposure of the LLZOto the ambient atmosphere.

In embodiments, the atmosphere exposed surface can be rehabilitated bytreating the affected surface with a suitable protic acid source (135)to form a membrane (140) having a protonated surface composition (e.g.,on one or more faces of the membrane, on two opposite faces of themembrane; or fully surrounding or encapsulating the LLZO (110) as shown)or, for example, comprised of an HLZO layer that can be distinguishedfrom the original minor amounts of HLZO in that the amount of protonexchange is substantially higher than the level of HLZO formed fromambient exposure. The HLZO layer formed from acid exposure can be called“super-protonated.” The super-protonated surface is less prone tofurther reaction with ambient moisture, which renders the surface stableagainst formation of a surface contamination layer (120) for a period oftime exceeding several hours, permitting a substantial working timeunder ambient conditions.

In embodiments, a pristine membrane (110) can be treated with a suitableprotic acid source (135) to directly form a membrane (140) having aprotonated surface composition (136) and without the appearance of LiOHand Li₂CO₃ (120) surface contaminants and HLZO (130).

In embodiments, at least a portion of the surface of a pristine membrane(110), for example, an LLZO or like membrane, can be contacted ortreated directly with a protic acid source (135) to form a membraneproduct (140) having bulk or interior composition (110), for example, anLLZO, and a protonated surface composition (136), for example, an HLZO,in the protic acid source contacted area(s). The product (140) andsimilarly contacted membranes are stabilized solid garnet electrolytematerials that can be further processed or handled in ambient atmosphereto make or assemble (145) (e.g., cell assembly) articles or devices thatinclude the surface protonated garnet electrolyte product (140).

In embodiments, an assembled cell (160) can include, for example, theabove mentioned surface protonated garnet electrolyte product (140), alithium metal source (150), and a glass tube or glass cylinder, or likevessel or separator (104), which combination permits lithium ion flux(155) from the lithium metal source (150) to and through the adjacentprotonated surface composition (136).

In embodiments, an assembled cell (160) can be cell tested (165) tomeasure the lithium ion flux and the longevity properties of theprotonated surface composition (140). In the cell test, the protonatedsurface composition (140), e.g., the HLZO surface layer(s) compositionis(are) substantially converted back into, i.e., regenerates, theoriginal lithiated surface composition (110), or nearly so, on the LLZObulk composition, which is in contact with a lithium ion source (150),or other like original composition, by the continuing Li⁺ flux from theLi metal during the test (170) according to the reaction, for example:

Li_((7-x))H_(x)La₃Zr₂O₁₂ +xLi⁺→Li₇La₃Zr₂O₁₂ +xH⁺

This reaction does not occur in an actual device unless one contacts theprotonated surface layer(s) composition with a solution having a highLi⁺ concentration, for example, greater than about 1 M Li⁺. However,such contact would regenerate the unstable LLZO or like garnetcomposition if accomplished in an ambient environment.

EXAMPLES

The following Example(s) demonstrate making, use, and analysis of thedisclosed article in accordance with the above general procedures.

Example 1 Fabrication of a Sintered Garnet Membrane

Gallium and niobium bi-doped compositions having a generalstoichiometric formula of:

Li_(7-z)La_(3-y)Ga_(y)Zr_(2-z)Nb_(z)O₁₂

where z is from 0.1 to 0.3, and y is from 0.1 to 0.3, was synthesized asfollows.

A suitable starting garnet powder was made from oxide precursors, suchas Li₂CO₃, LiOH, La₂O₃, ZrO₂, Ga₂O₃, Nb₂O₅, Al₂O₃, AlOOH, and likecomponents. The powders are thoroughly dry mixed with, for example,turbulent mixing for 30 to 60 min with ZrO₂ media, and wet mixed, forexample, in a liquid carrier such as isopropanol (IPA) or de-ionizedwater. The starting components are first combined using a vibratorymixer for 1 to 2 hrs until the mixed components de-agglomerate. Thebatch was then ball milled for about 1 to 2 hrs for further mixing. Toavoid significant interaction between the liquid carrier and precursorcomponents, a preferred total mixing time was, for example, less than 6hrs. Then, the resulting slurry was dried at 100° C. for 1 to 2 days.The dried powder was then ready for calcination. The calcinationtemperature depends on the composition. Low temperature compositions canbe dried at, for example, 1000° C. to 1100° C., whereas high temperaturecompositions can be up to 1200° C. The dried powder was calcined at1100° C. for 6 hrs to form a garnet structure having a cubic lithiumgarnet phase larger than 90%. The powder was then placed in a platinumcrucible with a refractory sheet cover such as pure alumina.

The garnet powder was then pressed without any addition to form drythick pellets. The thick pellet was sintered in air with platinumprotection to minimize the contact with air. The gallium and niobiumbi-doped lithium garnet has a low sintering temperature from 1000° C. to1200° C. depending on the doping levels, for example, each of galliumand niobium in the stoichiometric range of 0.1 to 0.5. A thin slice (ormembrane) was cut from the pellet having a diameter of 27 mm and athickness of 0.5 mm. The cut membranes are the specimens that were usedin the cell test (see Example 3 below). An example of these compositionsis Li_(6.75)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂, which is an exampleof low level gallium doping, for example, gallium in the stoichiometricrange of 0.1 to 0.5.

After calcination, the powder was milled by either dry ball or dry Jetmill. Ball milling was a very fast and low cost milling process. Afterball milling, the powder usually has a bi-modal distribution, such as0.5 to 0.8 microns and 6 to 8 microns, with no agglomerates. The jetmilling is a high energy process, and a very fine powder can be achievedby colliding particles or using an impact liner surface (such as ZrO₂,alumina, tungsten, etc.). Lithium garnet calls for a ZrO₂ liner. Afterjet milling, the powder usually has a mono-modal distribution with a D50around 0.5 to 0.8 microns. However, the jet milled powder tends to formsome agglomerates, and storage may require a sealed condition.

A garnet cylindrical solid can be made by dry pressing. It is preferredthat the powder has no agglomerates, so de-agglomeration may benecessary. First the powder was uniformly placed in the die and was thentapped. Tapping was called for because it ensures that the powdernaturally compacts. A vacuum die was then used to remove any largevoids. Tapping usually takes 1 to 5 mins to ensure complete compaction,and then pressed at a force at least 13,000 lbs, or greater than 7000psi for 2 to 4 mins.

Optimal sintering condition can depend on the composition. For disclosedcompositions, the sintering temperature can be, for example, from 1100°C. to 1180° C. for 2 to 15 hrs. This higher temperature and long holdingtime can significantly enhance the degree of sintering, although it isless economical and could cause grain growth. A preferred sinteringcondition was at a higher temperature but having a shorter holding time,or at a lower temperature but having a prolonged holding time. A typicalsintering condition was 1120° C. for 6 hrs for one composition, and1180° C. for 10 hrs for another composition. After sintering, thegeometric density was measured to ensure the density was greater than4.5 g/cc.

The cylindrical solid of lithium garnet was mechanically cut into a thindisk using a precision diamond blade wet saw. It was observed that diskquality improves with a mineral oil cutting medium and a high cuttingspeed. The finished disk thickness was from 0.4 to 0.8 mm and has a0.040 mm thickness variation across the disk. Further dry polishing ispossible to achieve a thinner disk. Thin membranes are then cut to 27 mmin diameter by laser cutting for discharge testing of the cell, and toprovide samples for ion conductivity.

In this example, the lithium garnet composition was of the formulaLi_(6.75)La_(2.9)Ga_(0.1)Zr_(1.75)Nb_(0.25)O₁₂ having bi-doping. Thepowder was milled by ball milling, and can have a bi-modal particle sizedistribution where the smaller size was around 0.6 to 0.7 microns, andthe larger size was around 7 to 8 microns. A bi-modal distribution wasfavorable for particle compaction during tapping.

The quantitative height results of the cylindrical solids range from 12mm (60 g) to 22 mm (80 g) after sintering. The diameter of the die was1.5″ (or 38.1 mm) and samples were explored at different sinteringconditions for optimizing density and shrinkage under variousconditions. An optimum sintering condition was about 1170 to 1180° C.for 6 to 15 hrs. A number of membranes, 0.5 mm thick, have been cut fromthese solids by diamond sawing. The quality of the slice can depend onthe green quality. Defects due to imperfect packing will be reflectedduring the sintering process as point or line defects. Quality, thinmembranes having flat, strong, hermetic and uniform thicknesses, can beachieved using the above mentioned preparative method. The ionconductivity of this composition was about 3 to 4×10⁻⁴ S/cm.

Example 2 Garnet Thin Membrane

A suitable starting garnet powder was made from oxide precursors, suchas Li₂CO₃, LiOH, La₂O₃, ZrO₂, Ga₂O₃, Nb₂O₅, Al₂O₃, AlOOH, and likecomponents. A solid state reaction can be used to form the garnetpowder. The solid state reaction involves precursor mixing andcalcination. This example has the composition ofLi_(6.75)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂, the calcinationtemperature was 1100° C. for 6 hrs. After calcination, the powder hadcubic lithium garnet phase over 90 wt %. The garnet powder was thendry-pressed to form a solid disk with diameter of about 38 mm and heightabout 20 to 40 mm. The sintering temperature was 1180° C. for 15 hrs.After sintering, the sample had the density of 4.78 g/cm³. The garnetsolid was then mechanically cut into a thin slice (or membrane) using aprecision diamond blade wet saw. The finished thickness was from 0.4 to0.8 mm and had a 0.040 mm thickness variation across the disk. The thinmembrane is flat, hermetic, and strong. The ion conductivity of theexample was 3.65×10⁻⁴ S/cm. For cell testing, the thin membranes werecut to 27 mm in diameter by laser. Further dry polishing is possible toachieve a thinner disk, such as less than 0.2 mm.

Example 3 Acid Surface Treatment of the Sintered Garnet Membrane

Extended acid treatment can result in substantial H⁺/Li⁺ exchange. Thegarnet surface, bulk, or both, can be “titrated” with a suitable acid toachieve a super-protonated surface or bulk composition. If the extent ofexchange is sufficient, the garnet no longer behaves as a base, i.e.,the aqueous pH no longer shifts to substantially basic when theprotonated material is immersed in neutral water. If the garnet issufficiently neutralized by acid, LiOH is not formed under ambientatmospheric exposure. Accordingly, the garnet no longer forms a lithiumcarbonate surface layer and can be freely handled in ambient conditions.

Prior to cell testing, one membrane was kept as a control, while anotherwas acid treated in accord with the disclosed method. In the acidtreatment both sides of the thin pellet were soaked in 0.01M HCl (pH 2)aqueous acid solution for 2.5 hrs, wiped dry with tissue paper, thenthoroughly rinsed with ethanol, and quickly blotted dry with tissuepaper in air. The control sample was only wiped with an ethanolsaturated tissue paper and air dried. The soaking solution was testedafter sample removal to measure the solution acidity of pH 2.

FIG. 5 shows a TGA trace comparing an acid untreated garnet control(500) sample and an acid treated garnet (510) sample after ambientatmosphere and moisture exposure. The TGA conditions are 2° C./min to900° C. hold for 2 hrs and then cool to RT under flowing air. Thecontrol sample (500) shows a two-step weight loss having one stepbeginning below 200° C. and another step above 700° C. The acid-soakedsample shows principally one step near 500° C. The weight loss of thecontrol sample may be attributable to adsorbed water below 200° C., andto decomposition of lithium carbonate over 700° C. The acid soakedsample weight loss near 500° C. may be attributed to loss of water fromthe protonated garnet, resulting in phase decomposition. As expected nocarbonate loss was found for the treated sample. The TGA data indicatethe significance of avoiding high temperature treatment for theproton-exchanged garnets; even conventional drying temperatures can leadto decomposition through water loss.

Based on the weight change attributed to water loss, the nominalcomposition of the acid-soaked sample wasLi₅H_(1.75)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O12.

Example 4 Cell Testing of Treated and Untreated Garnet Membranes

Cell testing was accomplished as follows. Aceramic-electrolyte-protected lithium metal anode cell was assembled inthe cell configuration shown in FIG. 2, which configuration resemblesthe scheme previously reported (see Visco, S. J., et al., Lithium-Air,ELC Encyclopedia of electrochemical power sources, Elsevier, 2009,4:376-383), with the exception that no microporous polypropylene layeris present between the lithium metal anode (150) and the LLZO solidelectrolyte. In the disclosed cell configuration, the lithium metal wasallowed to contact the garnet pellet directly without any separatorlayer present in between. A mixed solution of LiPF₆ in ethylenecarbonate (EC) and dimethyl carbonate (DMC) (EC/MDC 1/1 v/v, BASF) wasused as anolyte. A mixed solution of 2M aqueous LiCl and 3M aqueousNH₄Cl was used as catholyte.

FIG. 2 is a schematic in cross-section showing a test cell assembly(200) incorporating a lithium metal anode (150) in contact with a solidgarnet electrolyte (110), the solid garnet electrolyte (110) furthercontacts an aqueous catholyte (230).

The test cell assembly (200) can include, for example;

-   a container (202) or like vessel;-   a glass tube or glass cylinder, or like vessel (104) surmounting the    solid garnet electrolyte (110) and further straddling the lithium    metal anode (150);-   a metal rod (206) traverses a seal and holder member (207) situated    on the end of the glass vessel (104), and the metal rod contacts an    organic anolyte (260) (such as LiPF6 in organic liquids ED/DMC; 1:1    by volume) in the interior of the glass vessel (104);-   a current collector (208) is attached to the external end of the    metal rod (206);-   a Ni foil (212) is attached to the internal end of the metal rod    (206);-   the Ni foil (212) is attached to or is in electrical contact with    lithium metal anode (150);-   the cell is further equipped with cathodes (240) supported by, for    example, the walls of the container (202), and a Ag/AgCl reference    electrode (210) in contact with the aqueous catholyte (230).

FIG. 3 compares an operating cell voltage for acid treated garnetpellets (diamonds, 310) and untreated garnet pellets (squares, 300)(control) as a function of current density. The improved performance ofthe acid treated garnet sample was apparent from the data shown in FIG.3, where the acid treated sample shows much lower polarization comparedto the untreated sample. The difference in performance is attributed tothe removal of the insulating lithium carbonate surface film in the acidtreated sample. When the current density of control garnet cell wasincreased to 0.1 mA/cm², the cell over polarized immediately within 1second. In contrast, the cell including the disclosed acid treatedgarnet pellet ran stably at current densities of 0.1 mA/cm², 0.5 mA/cm²,and even 1.0 mA/cm².

FIG. 4 shows an exemplary operating cell voltage over time for an acidtreated garnet pellet at high current density of 1.0 mA/cm² for 70+ hrssuch as almost 100 hrs.

The disclosure has been described with reference to various specificembodiments and techniques. However, it should be understood that manyvariations and modifications are possible while remaining within thescope of the disclosure.

What is claimed:
 1. A solid garnet composition, comprising: a bulkcomposition comprising a lithium garnet; and a surface compositioncomprising a protonated garnet on at least a portion of the exteriorsurface of the lithium garnet, wherein the surface protonated portion ofthe garnet is compositionally stable in air and insensitive to carbondioxide.
 2. The composition of claim 1 wherein the lithium garnet isLi₇La₃Zr₂O₁₂ and the protonated garnet is Li_((7-x))H_(x)La₃Zr₂O₁₂,where x is from 0.1 to
 7. 3. The composition of claim 1 wherein thelithium garnet is: Li_(6.75)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂ andthe protonated garnet isLi_((6.75-x))H_(x)La_(2.9)Ga_(0.1)Nb_(0.25)Zr_(1.75)O₁₂, where x is from0.1 to 6.75.
 4. The composition of claim 1 wherein the protonated garneton at least a portion of the exterior surface of the lithium garnetcomprises the entire exterior surface of the lithium garnet.
 5. Thecomposition of claim 1 wherein the protonated garnet on at least aportion of the exterior surface of the lithium garnet comprises a layerhaving a thickness of from 0.1 to 100 nm.
 6. The composition of claim 1wherein the surface composition has a lithium concentration of from 0.1to 46 mol %.
 7. The composition of claim 1 wherein the surfacecomposition is substantially free of an electrically insulating Lisurface species.
 8. The composition of claim 1 wherein the surfacecomposition comprises a surface free of at least one of LiOH, Li₂CO₃, ora combination thereof.
 9. A composite electrolyte structure, comprising:a protected anode structure comprising: a first layer comprising aceramic electrolyte comprising the solid garnet composition of claim 1;a second layer comprising a source of lithium metal, wherein the firstlayer and the second layer are in physical contact; and a cathode. 10.The composite electrolyte structure of claim 9 further comprising anaqueous catholyte situated between and in contact with the protectedanode structure and the cathode.
 11. The composite electrolyte structureof claim 9 wherein the cathode is a lithium cobalt oxide compound.
 12. Amethod of making the solid garnet composition of claim 1, comprising:contacting an air sensitive lithium containing garnet with a protic acidto form the solid garnet composition having a protonated garnet surfacecomposition.
 13. The method of claim 12 wherein the protic acid isselected from at least one of: a mineral acid, an organic acid, or acombination thereof.
 14. The method of claim 12 further comprisingcontacting the air stable and carbon dioxide insensitive solid garnetcomposition having the protonated garnet surface with a source oflithium ions to regenerate the air sensitive lithium containing solidgarnet.
 15. The method of claim 14 wherein the contacting with a sourceof lithium ions is accomplished in a sealed cell structure.
 16. Themethod of claim 14 wherein the source of lithium ions is lithium metal.17. A passivation method comprising: ion exchanging the lithium ions onthe surface of an air sensitive lithium containing solid garnet membranewith protons to form an air stable and a carbon dioxide insensitivesolid garnet electrolyte having a lithium-depleted protonated surface.